Geophysical exploration using EM fields is an established technique for determining whether or not a subsurface may contain a hydrocarbon-containing reservoir. Such a technique may be referred to as Sea Bed Logging (SBL) or, more generally, controlled source electromagnetic measurement (CSEM). WO 01/57555 describes basic aspects of the SBL technique.
EM exploration is a stand-alone technique providing a direct indicator for hydrocarbon reservoirs, and is complementary to seismic methods of exploration. By combining both seismic and EM methods, reservoirs can be identified with improved probability compared to one method alone.
Today, EM surveys are normally done by deploying a number of vector magnetic and electric field receivers known as nodes on the sea bed to form a two dimensional array. By towing an EM transmitter close to the sea bed, in proximity to the receiver array, electrical impedance variations in the subsurface beneath the sea bed can be detected by the receiver system. Data from the receivers can then be processed to obtain information about the subsurface.
While the description in this specification mentions the sea and sea bed, it is to be understood that these terms are intended to include inland marine systems such as lakes, river deltas, etc.
EM receivers for use in survey systems should be as sensitive as possible, in order to obtain as much information as possible about the subsurface. In particular survey situations or types of survey, receiver sensitivity is particularly important. There is therefore a need to improve the sensitivity of EM receivers.
It is therefore an object of the present invention to provide EM receivers suitable for sea bed logging, which are more sensitive than currently existing sensors based on conventional technology.